This invention relates to a multilayer bonding prepreg comprising a fluoropolymer, a reinforcement typically consisting of fiberglass to reinforce the low signal loss fluoropolymer, a surface coated thermosetting resin and a ceramic dielectric modifier to control the coefficient of thermal expansion and reduce the resulting dissipation factor. This invention further relates to a bonding ply composition that has the ability to reform during lamination such that the PTFE-fiberglass layer has the ability to conform to the outline of copper circuitry, thus reducing the mass of thermosetting material required to fill circuitry. It has been unexpectedly found that one can influence the ability of the PTFE-fiberglass layer to conform around copper circuitry by designing a thermosetting adhesive composition of very low viscosity. Ceramic is used in either the fluoropolymer coated glass component, the thermosetting resin surface component, or in both components. The composite is used as a low signal loss bonding ply, Df less than 0.005, Dk less than 4.0, that can be pressed at low temperatures to manufacture a multilayer circuit board for high frequency applications. The invention also relates to a copper clad laminate that may comprise the following, a low loss prepreg (fluoropolymer-reinforcement-thermosetting resin), another low loss substrate (fluoropolymer-reinforcement), a reinforcement layer that may or may not contain a thermosetting resin, a film that may or may not contain fluorine.
In the electronics industry multilayer circuit boards are prepared by bonding a layer of incompletely cured thermosetting resin reinforced with fiberglass between layers of a fully cured print and etched laminate. For a multilayer epoxy based circuit board, first an epoxy coated fiberglass composite is laminated with thin copper foil on both sides. The copper is patterned using conventional printed circuit board manufacturing processes. This layer is referred to as the inner layer. These innerlayer print and etched copper laminates are then bonded together typically using an FR-4 prepreg (a flame retarded partially cured sheet of epoxy coated fiberglass that has no copper foil cladding) using the partially cured epoxy as an adhesive layer by pressing the construction together in a press at temperatures such as 360xc2x0 F. (182xc2x0 C.) for two hours at 200 psi, thereby fully curing the epoxy FR-4 adhesive layer. A composite is thereby created having non-pattered copper layers at the surfaces and patterned inner layers being separated by the adhesive layer. The top and bottom non-patterned copper layers (the outer layers) can then be print and etched yielding a multilayer circuit board.
One drawback of using many conventional thermosetting resins as the adhesive layer is the poor electrical properties of the bonding adhesive layer. Epoxy based thermosetting resin, for example, has poor electrical loss characteristics in the 1-100 gigahertz range. For very long trace lengths, signal degradation forces the use of lower loss dielectrics. This is increasingly becoming the case for high speed digital applications (routers, backplanes, motherboards and daughter boards). For the RF and mm wave frequencies, polytetrafluoroethylene (PTFE) based materials are traditionally used to prevent signal loss. PTFE based materials have been available for a long time for the most demanding low signal loss applications but have been avoided for cost considerations. Conventional thermosetting resins have too high a loss tangent at the high frequencies and are nearing their ultimate limits at 2.5 GHz. As frequencies extend to the 5 and 10 GHz range, it is likely that epoxy resins will be replaced by higher performing materials. In the last 10 years epoxies were acceptable up to 2-3 GHz but seem to be being designed out as designs move to 5 GHz. Suppliers of epoxy laminate have been reducing the loss tangent of their products by switching to lower loss polyphenylene oxide based polymers and ceramic fillers. Typical fiberglass based PTFE products have 0.001-0.004 loss tangents, depending on fiberglass wt %, versus 0.007-0.014 for modified epoxies and related materials (10 GHz). As signal integrity drives the use of higher performing materials, epoxy based solutions will eventually fall short even with high loadings of ceramics or the addition of lower loss modifying resins.
The dielectric constant is less critical but it is desired to be below 4.0. At the high frequencies the market is largely driven by dissipation factor and the dielectric constant is taken for granted. Backpanels and daughter cards have been growing in the number of layers due to the need to eliminate crosstalk between circuitry. Lower dielectric constants lead to thinner dielectric spacings. By designing using lower and lower dielectric constant, the engineer can increase the number of layers yet not increase the total overall pwb thickness if the dielectric thickness of the individual layers can be reduced by using lower dielectric constant materials.
An alternative solution is the use of expanded PTFE that has been filled with epoxy and ceramic, thereby diluting the concentration of the higher loss epoxy component. This combination of epoxy, ceramic, and PTFE results in a sufficiently low loss product to be acceptable for high speed digital applications. The downside is that the expanded PTFE based solution is quite expensive and there are issues of dimensional movement that becomes significant with increasing layer count. U.S. Pat. Nos. 4,985,296; 4,996,097; 5,538,756; and 5,512,360 awarded to W. L. Gore describe the use of a thermosetting resin impregnated into an expanded PTFE web. These patents teach the use of incorporating ceramic in the PTFE expanded web manufacture and/or part of the non-fluorinated adhesive resin system to obtain low loss materials.
Ceramic filled resin systems based on polybutadiene-woven fiberglass based prepregs, both filled and unfilled with flame retardant additives, are known to be relatively low loss materials (U.S. Pat. No. 5,571,609). These materials suffer from the inconsistent quality of the peroxy cured rubber system and the poor bond strengths of the cured rubber to copper foil. A related material, crosslinked polyesters filled with kaolin, has attractive dielectric properties but unattractive peel strengths and other fabrication problems.
Polyphenylene oxide (PPO, APPE, PPE) based resin systems that are cured systems of low molecular weight PPO and epoxy resins have some process limitations (U.S. Pat. No. 5,043,367; 5,001,010; 5,162,450) for high-speed digital or high frequency applications. Their loss tangents in the gigahertz frequency range are reported to be in the 0.006-0.008 range. This is an improvement over standard epoxy but their lack of flow has led to their withdrawal from the marketplace.
Very low loss solutions include PTFE based materials and optical interconnects. Solutions containing pure PTFE based adhesive layers have the disadvantage that these materials need to be processed at temperatures exceeding 700xc2x0 F. (fusion bonding, 371xc2x0 C.). There are fabricators today building multilayer structures based on fluorinated resin systems. Most fabricators do not have equipment capable of pressing at these temperatures, nor are the extended heating and cooling attractive to fabricators. High temperature pressing on a 34-layer count stack up could result in decreased reliability of plated through holes, PCB warping, and copper pad distortion. In high speed digital applications, via holes and stubs are a real source of signal loss. The number one obstacle for high speed digital applications is the high layer count stack-up that encourages OEMs to source board materials that are process friendly. For high speed digital applications, the high frequency materials may be separated from the standard FR-4 lower frequency layers. This may lead to multiple lamination cycles. Fabricators prefer to press laminates relatively quickly at conventional epoxy pressing temperatures below 350xc2x0 F. (177xc2x0 C.) and have scaled their pressing capacity so that it is not a bottleneck in the entire printed circuit board fabrication process. Thus FR-4 is a material of choice. However, increasing operating frequencies demand materials having lower loss characteristics. Therefore, a composite that enables multilayer lamination at epoxy processing temperatures that has a minimum component of a hydrocarbon resin is especially desirable.
One approach is to combine a PTFE-fiberglass composite with a traditional FR-4 epoxy impregnated fiberglass within a laminate or between two layers of copper. This has been described in WO0011747. The advantage of this approach is that it is not necessary to treat the surface of the PTFE-fiberglass with a layer of thermosetting adhesive. The disadvantage of this approach is that the thermosetting resin is combined with a fairly lossy fiberglass reinforcement. Because adhesion to the copper is required at the processing temperatures of conventional thermosetting resins, an FR-4 layer would be required between the copper layer and the PTFE-fiberglass layer. The simplest embodiment would comprise a copper layer, FR-4 layer for bonding, PTFE-fiberglass, FR-4 layer, copper. This approach would be somewhat challenged to obtain a thin core laminate reaching very low dissipation factors and 3-5 mil dielectric spacings.
Disclosed in this invention is a fluoropolymer coated fiberglass composite that is used as the component to deliver low signal loss properties. The fluoropolymer coated fiberglass composite is then surface treated to enable it to bond to other surfaces. Surface treatment is conducted on the nanometer scale in order to maintain the desirable bulk properties of the fluoropolymer. The surfaces of the chemically modified sheet of fluoropolymer-coated glass are then coated with a thin layer of a thermosetting resin which may or may not contain a ceramic dielectric modifier or other filler (refer to FIG. 1). Although the thermosetting resin may represent a compromise to the otherwise good electric properties of the PTFE coated fiberglass, the thermoset enables the manufacture of a multilayer laminate at conventional epoxy processing temperatures. The thermosetting resin is cured to a minimum extent possible (B-staged) during the coating of the thermoset onto a fluoropolymer composite comprising a substrate selected from woven fabric, non-woven or a polymeric film. It is preferred that the thermosetting layer is dried onto the low loss substrate with the ideal condition being no degree of cure. This is limited in a real life manufacturing sense because the solvent must be driven from the prepreg in an economically viable fashion that requires forced hot air to drive the solvent off to a lower limit that is acceptable by IPC standards. The electrical properties of the resulting prepreg are then determined by the ratio of the coated thermosetting resin to the fluoropolymer-coated fiberglass starting material. It is preferred to limit the amount of thermosetting resin to just enough to fill the spaces between the copper traces of the inner layers and still obtain a good bond. When the low loss substrate comprises PTFE, fiberglass, and/or a ceramic additive, no flame retardant is required ensuring a low dissipation factor at high frequencies.
One significant challenge is to obtain a low dielectric constant, low dissipation factor, low cost, and good thermal reliability. Japanese unexamined patent application 60-235844 teaches a composition comprising a PTFE woven glass substrate. However, no regard for the dissipation factor of the composition is given. 10 years ago most thermosetting resins would satisfy the dissipation factor for the low frequencies in use. Combining a PTFE-fiberglass substrate with a thermosetting resin requires a special attention to the gap filling ability of the prepreg to fill gaps between print and etched circuitry, the flow rheology of the thermoset to fill those spaces, the loss factor (Df) desired by the designers, and therefore the balance between the ratio of the PTFE-fiberglass layer, the low loss substrate layer, to the thermosetting layer. A given amount of thermosetting resin is necessary to fill one and 2 oz circuitry. At least a mil of thermosetting resin is necessary to fill 1 oz circuitry and about 2.0 mil is necessary to fill 2 oz circuitry. For a composition where a thermosetting resin is deposited onto the two faces of a low loss substrate that has no porosity, only thermosetting resin that is deposited on one face of the substrate is capable of filling gaps in copper circuitry on that respective side. In other words, it cannot be anticipated that thermosetting resin would flow through a non porous low loss substrate to fill gaps in copper circuitry on the opposite side of the low loss substrate. Because a flame retardant thermosetting layer can be anticipated to be the highest loss component, the careful choice of a low dielectric constant ceramic modifier will help reduce the dissipation factor.
High speed digital applications demand thin substrates with dielectric constants less than 4.0, dissipation factors closing in on 0.005, dielectric spacings of 3 to 4 mil, and thermally reliable substrates. FR-4, for example has a dissipation factor of 0.018 at 10 GHz. A construction of 1 mil of FR-4, for example, disposed on each side of a chemically modified 2 mil PTFE-fiberglass substrate, would be expected to have a dissipation factor in excess of 0.01. For such a composition to reach a dissipation factor of 0.005 or less, the chemical structure of the thermosetting layer must be carefully chosen, and the balance between the very low loss PTFE-fiberglass layer and the higher loss thermosetting layer, must be carefully controlled. Prior art must be carefully evaluated as to whether it meets the requirements of a dissipation factor less than or about 0.005, and prescribe what is required. For a given PTFE-fiberglass laminate, the dissipation factor of the laminate will be dependent on the concentration of the high loss fiberglass. However, if the PTFE concentration is kept constant and the high loss fiberglass is replaced with a low loss ceramic material of suitable dielectric constant, a reduction in dissipation factor can be achieved. WO0011747 and other prior art do not suggest that the dissipation factor of the composition can be reduced by incorporating low loss, low dielectric constant ceramic additives to the thermosetting layer. It also does not suggest a process of combining a low loss substrate (PTFE-fiberglass) or plurality of such prepregs with a prepreg comprising a fluoropolymer, a reinforcement, and a thermosetting resin, or plurality of such prepregs to obtain a prepreg layer or a copper clad laminate. By alternating layers of prepregs comprising a thermosetting resin and layers of prepreg comprising only reinforced fluoropolymer (PTFE-fiberglass), lower cost can be obtained in addition to a reduced dissipation factor.
Unexamined Japanese Patent Application 6-322157 suggests the use of filler materials to achieve a high dielectric constant composite comprising a narrowly defined epoxy resin, a reinforcement, and high dielectric constant fillers. 100 to 600 phr of high dielectric ceramic powder combined with 100 phr of a polysulfone is used to impregnate a reinforcement. Dielectric constants greater than 7.4 are taught. This prior art does anticipate the addition of high dielectric constant ceramics to a layer of epoxy resin but does not disclose a solution for a low loss laminate or prepreg, the lowest dissipation factor cited for the compositions cited as 0.011.
The present invention anticipates a composition comprising a fluoropolymer, a reinforcement, and a thermosetting layer that is capable of producing a laminate with a dissipation factor less than 0.005 and a dielectric constant less than 4.0. It has been unexpectedly found that one can influence the ability of the PTFE-fiberglass layer to conform around copper circuitry by designing a thermosetting adhesive composition of very low viscosity. The combination of a thermosetting resin, a reinforcement, and a fluoropolymer is referred to as the low loss substrate. This low loss substrate or plurality of substrates can be combined with another low loss substrate such as PTFE-fiberglass, fluoropolymer film, or FR-4 epoxy-fiberglass prepreg in any combination or plurality of combinations, or a reinforcement such as fiberglass that may or may not comprise a thermoplastic or thermosetting resin, to yield a prepreg layer or a copper clad laminate.
In one aspect, the present invention relates to a process for fabricating a low loss multilayer printed circuit board using a bonding ply comprising a fluoropolymer substrate and a thermosetting adhesive composition. The fluoropolymer composite comprises at least one fluoropolymer and a substrate selected from fluoropolymer impregnated woven fabrics, nonwoven fabrics and polymeric films. The resulting electrical properties have a dissipation factor less than 0.005 and a dielectric constant less than 4.0.
In a further embodiment, the invention relates to a low loss bonding ply that unexpectedly reforms during lamination to conform to the outline of print and etched circuitry such that less mass of thermosetting resin is required to fill gaps in copper circuitry. This ability to reform during lamination is enabled by a high flowing thermosetting resin system. This ability to conform to the outline of circuitry is not limited to the thermosetting resin system. The PTFE-fiberglass layer unexpectedly participates in gap filling by reforming around circuitry.
In another aspect, the invention relates to a multilayer printed circuit board comprising a plurality of printed circuit board layers bonded together by means of the same bonding ply.
In yet another aspect, the invention relates to a composition comprising a fluoropolymer composite comprising at least one fluoropolymer and a substrate selected from fluoropolymer impregnated woven fabrics, nonwoven fabrics and polymeric films; and a thermosetting adhesive composition that is combined to form a prepreg or a copper clad laminate.
In another aspect, the invention comprises any combination of a low loss substrate containing a fluoropolymer and a reinforcement with a thermosetting resin that may or may not be reinforced to form a prepreg or a copper clad laminate.
In another embodiment, the invention further comprises combining a prepreg that could be a low loss substrate such as PTFE-fiberglass, a polymeric film such as skived PTFE, a low loss substrate such as PTFE-fiberglass-thermosetting resin system, or a high loss substrate or prepreg such as FR-4 epoxy-fiberglass to form a copper clad laminate or a prepreg consisting of any one of the above or a plurality of the above to form a multilayered prepreg serving as one prepreg or to form a copper clad laminate.
In a further aspect, any one of the low loss or high loss substrates alone or in combination may have some level of porosity due to laser ablation and be combined in any fashion as above to form a combined prepreg layer.